In recent years, by the development of remote sensing technologies, observational instruments mounted in aircraft and artificial satellites have grown in performance, and the amount of information transmitted from the air to the ground is increasing. In order to efficiently transmit, to the ground, data to be generated in observation instruments having more improved performance in the future, a free space optics (FSO) system that uses an optical frequency band, by which a wider bandwidth can be expected more considerably than by microwaves, has been studied.
In order to achieve a large-capacity free space optics (FSO) system, it is necessary to employ a high-speed technology for a bit rate of a transmission signal and a wavelength multiplexing technology. In the free space optics (FSO) system that performs an ultra-long distance transmission from an artificial satellite and the like to the ground, a highly sensitive receiver is required because the signal light is largely attenuated due to propagation through the free space. In this case, it is efficient to apply a common technology with an optical fiber communication technology, that is, an optical transmitting and receiving technology using a single mode fiber (SMF). The reason is that it is possible to use a direct optical amplification technology with low noise and high gain, a highly sensitive digital coherent receiving technology, a high bit rate transmitting and receiving technology, a dense wavelength division multiplexing (DWDM) technology and the like, for example.
Up to now, such a free space optics (FSO) system applying a technology using a single mode fiber (SMF) has been developed. In the free space optics (FSO) system applying the technology using a single mode fiber (SMF), it is necessary to couple laser light to a single mode fiber (SMF) with a small core diameter.
In the FSO system that transmits the signal light over a long distance from an artificial satellite and the like, a free space optics (FSO) receiver needs to have a telescope with a large opening size in order to focus light having sufficient optical power. In this case, because the diameter of the telescope is several times as large as a spatial coherence radius of the laser light that has been propagated through the atmosphere, the laser light becomes susceptible to atmospheric turbulence such as wind. Consequently, there has been a problem that the degree of fluctuation of beam spots of the laser light focused by the telescope becomes large.
At this time, there is a problem that it is difficult to communicate stably because the intensity of laser light incident upon an optical receiver fluctuates a lot due to the fluctuation of the beam spots. Particularly, when strong damping due to large intensity fluctuation (fade) arises, an error or a loss of received data occurs; as a result, the overhead of an error-correction code (Forward Error Correction (FEC)) increases, or a retransmission process is required. This causes the effective throughput of the free space optics (FSO) system to decrease.
As described above, there has been the problem in the free space optics (FSO) system that communications become unstable due to a wave-front distortion that the laser light suffers during the atmospheric propagation.
A technique to solve the problem is disclosed in Patent Literature 1. A related free space optics (FSO) receiver described in Patent Literature 1 includes a telescopic collection system, a wavelength demultiplexer, photodetectors, analog-to-digital converters, and a digital signal processor. The free space optics (FSO) receiver has a configuration in which the light is collected from the wavelength demultiplexer into a plurality of individual fiber end faces, and using a tapered fiber bundle or a tapered single fiber, signal light is concentrated into a single output fiber for input to the photodetector.
The related free space optics (FSO) receiver is configured to collect light, using a collection lens, onto a large-aperture surface of a multimode fiber bundle that is obtained by fusing single mode fibers. This makes it possible to achieve stable fiber coupling for fluctuation of beam spots. In addition, a core diameter is tapered to be thinner to a diameter equal to that of a single mode fiber (SMF), which enables coupled laser light beams to be concentrated at the core of the SMF and to connect with optical components adapted to the SMF in a subsequent stage.